Flash sintering is a novel sintering method, which has the advantages of short sintering time and low energy consumption. In previous study, yttrium-stabilized zirconia can be sintered at room temperature and noticed that the sample starts sintering after the arc appears on the sample’s surface. But the mechanism of how the arc affects flash sintering is not clear. An electric and thermal coupling model of the arc and sample was established to analyze the effect of the arc on flash sintering. It has been found that the heat conducted by the arc to sample is the main cause of flash sintering and the closer the arc is to the surface of the sample, the more likely the sample is to start flash sintering.